Piezoelectric materials offer coupling between mechanical and electrical energy, allowing them to be effectively fabricated as transducers that can be configured as either sensors or actuators. Many sensing applications incorporate piezoelectric materials, because they require minimal signal conditioning and have wide bandwidth well into the MHz range. Furthermore, piezoelectric materials have been found particularly well suited for micro-electromechanical systems (MEMS), as the energy density does not decrease with the dimensions, as is the case in electromechanical or magneto-mechanical systems. When applied in MEMS applications, piezoceramics are typically constrained to a thin film, which places certain limits on the design of the device and typically requires the use of the lower k31 piezoelectric coefficient (1). An alternative configuration for sensors is vertically aligned piezoelectric nanowire (NW) arrays that allow for facile interfacing with electrical interconnects. Piezoelectric nanowires have evoked tremendous curiosity in the field of nanotechnology for energy applications primarily due to their excellent electro-mechanical energy conversion capabilities which are unchanged as the scale is reduced and, in addition to their ability to be utilized in advanced sensors, they can function as sufficient power sources for certain low power wireless electronic devices and miniature autonomous systems [1,2]. Power generating nano-electro-mechanical system (NEMs) fabricated using piezoelectric nanowires have become renowned in the research community, as they are able to convert several different sources of mechanical energy into electric power, such as: sound waves [3]; ultrasonic waves [4-6]; vibrational energy [7,8]; atomic force microscope tip induced stimuli [9,10]; and biomechanical energy [11,12].
The power generating capacity of devices based on aligned piezoelectric ZnO nanowire (NWs) arrays has been studied rigorously and it has been reported that the energy conversion efficiency of such a device is sufficiently high for production of electricity that can potentially power nanosystems [9]. The direct piezoelectric effect responsible for the energy-harvesting behavior is identical to the response required for sensing. However, energy harvesting represents a more simplistic operation, as the voltage output can contain significant noise, requires little to no correlation to the input energy and places no limits on the bandwidth or stability of the response. On the contrary, a functional sensor must produce an output that can be very accurately correlated to the force (mechanical measurands) acting on it and without noise that would limit the sensitivity and measurement floor. Among the piezoelectric NEMs, those made of ferroelectric perovskite nanostructures and thin films such as PZT (PbZrxTi1-xO3) [8, 13-15], and Barium Titanate (BaTiO3) [16-18] can produce greater energy due to their higher electro-mechanical coupling coefficients and thereby, provide an efficient means to harvest mechanical energy. The NW form of these materials offers considerable advantages due to the high aspect ratio, which leads to highly deformable structures [19, 20] and size effects [21] that act to enhance the piezoelectricity of the ceramic. Consequently, piezoelectric NWs have tremendous potential to be applied in the emerging field of nano-electromechanical systems (NEMS).
However, environmental concerns over the use of lead based piezoelectric materials have enhanced the need to develop and utilize lead-free BaTiO3 nanostructures. Moreover, prior to this study the synthesis of vertically aligned arrays of BaTiO3 nanowires (NWs) had not been developed and thus this high performance lead free composition has received little attention. Previously, Wang et al. [19] performed a numerical analysis to show that the BaTiO3 NWs have higher power generating capability as compared to ZnO NWs for the same size. Recently, Wang et al. [20] applied ZnO NWs as vibration sensors to detect the resonance characteristics of a cantilever beam and evaluated the voltage generating performance. Although ZnO NWs have garnered significant interest for sensing and energy harvesting, a low piezoelectric coupling coefficient and semiconductor behavior are unlike many ferroelectric ceramics and, therefore, sensors therefrom show low sensitivity and a high noise floor [22]. Although ZnO NWs have a low dielectric constant, which increases its voltage output, the performance has been very limited and no sensor has been demonstrated to produce a high coherence between the input and output across the sensor's bandwidth [3, 23-25]. Ferroelectric perovskite nanostructures such as PZT (PbZrxTi1-xO3) [13, 26, 27] NWs improve the electromechanical coupling performance of NW-based devices; however, they have only been applied for energy harvesting applications as the environmental concerns with lead-based piezoelectric materials encourages the use of lead-free piezoelectric nanostructures for sensors [28]. Among lead-free ceramics, barium titanate (BaTiO3) possesses one of the highest coupling values. However, no synthesis method for the growth of vertically aligned BaTiO3 NW arrays has been demonstrated prior to the inventors' efforts, and thus this high-performance lead-free material has received little attention in the NW form. Herein the preparation of ultra-long and short BaTiO3 nanowires by two methods and their use for the preparation of sensors and energy harvesting devices